One of the most important phenomena impacting on nuclear power plant reliability in recent years is stress-corrosion cracking of piping systems in boiling water reactors. The problem results from a combination of stress and oxidation in the heat-effected zone of welds in boiling water reactor piping systems. Extending about 0.1 inches on either side of the weld, the heat-effected zone is highly susceptible to cracking because of the high level of residual stresses resulting from heating of the metal to temperatures near its melting point. Once initiated, stress-corrosion cracks are propagated through the pipe wall by the tensile stress acting on the metal. In several cases, cracking has extended completely through the wall, allowing reactor cooling water to seep out of the system, with potentially disastrous results.
Weld-overlay repair is an effective interim repair technique for welds affected by intergranular stress-corrosion cracking. It not only places the pipe interior in compression, but also adds strength by providing a barrier to through-wall crack propagation. Weld-overlay repair consists of applying weld beads to the outer surface of the pipe in the area of the affected weld. The overlay is applied with the tungsten inert-gas process, which is proven to be the most reliable method for achieving defect-free weldments. The weld beads are applied circumferentially and are overlapped to form a sound weld buildup extending n both sides of the existing weld.
In order to allow accurate modeling and calculation of the residual-stress pattern, making it possible to confirm the effectiveness of the completed repair, automatic weld overlay techniques are desirable. In addition, the application of weld overlays to piping systems in boiling water reactors takes place in areas exposed to nuclear radiation from activated components in the vicinity. Thus, in order to reduce unnecessary exposure of maintenance personnel to the radiation, the weld overlay apparatus must not only be automatic but also quickly installable and operable by remote control from a control area safely removed from the contaminated area.
Among the points in piping systems of boiling water reactors most susceptible to stress-related cracking is the juncture between the substantially horizontal recirculating system ring header and the vertical risers projecting upwardly therefrom. These junctures, known as sweepolets, are saddle-shaped orthogonal joints between the large 22" diameter cylindrical header pipe and the smaller 10" diameter cylindrical riser. While the overlay repair technique has been applied successfully to essentially two-dimensional welds such as straight-pipe butt welds and fittings such as elbows and tees, the three-dimensional saddle shape of the header-to-riser arrangement introduces complications from the standpoint of stress analysis and remote application of the weld overlay.
Accordingly, there is a need to provide an automatic welding apparatus which can be quickly attached to a sweepolet joint and operated remotely from the radioactively contaminated area which permits weld-overlay repair of the saddle-shaped sweepolet weld.
Apparatus for automatically guiding a tool such as a weld head or a blow torch around a track are known in the art. For example, U.S. Pat. No. 2,705,629 discloses a flat, band-like track supported above the workpiece, and a carriage which travels around the track for joining two sections of a pipe together. However, that track is designed to provide carriage movement in a single plane, and hence cannot be adapted to the saddle-shaped weld of a sweepolet.
U.S. Pat. No. 4,260,869 discloses a traveling carriage apparatus adapted to travel along a track disposed about or upon a workpiece to carry a weld head. This device provides the additional advantages of three hinged-together portions to compensate for radius changes in a curved workpiece. Again, however, the track can follow a curved contour but only in a single plane.
Accordingly, there is a need to provide a track-mounted welding apparatus which can follow a predetermined path in two orthogonal planes.